Fast cooking converted rice

ABSTRACT

The invention concerns the use of rice having an amylose content less than 15 wt. % for preparing converted rice capable of being cooked in five minutes.

Steamed rice is a rice that has been subjected to steeping, thermaltreatment, drying and, often, machining. This treatment permits, inparticular, a non-sticky rice to be obtained, the grains of which areintact, but with the drawback that the cooking-time is lengthy. Theinvention overcomes this drawback by virtue of a steamed rice having agreatly shortened cooking-time.

The varieties of rice having an amylose content lower than 15%, relativeto dry matter, include, in particular, perfumed varieties of rice fromThailand having an amylose content generally amounting to between 10%and 15%, which hitherto have been sold as they are, just blanched inorder better to preserve all their aroma. There are many other, lessexpensive varieties of rice for steaming.

In accordance with the invention, steaming is carried out in respect ofrice having an amylose content lower than 15% by weight, relative to drymatter, amounting in particular to between 10% and 15%.

It has been found, unexpectedly, that steamed rice having an amylosecontent lower than 15% by weight, relative to dry matter, whether it becargo rice or paddy rice, can be cooked in 5 minutes after beingsubjected to steaming. Varieties of rice having an amylose content lowerthan 10% are known which are also suitable for the invention.

Steamed rice that is suitable for rapid cooking can preferably beprepared by steeping the rice having an amylose content lower than 15%by weight in water at a temperature lower than 70° C., in particularbetween 40 and 70° C., under such conditions and for such a time that,after removal of the surface water by drainage or spin-drying, steepedrice is obtained having a moisture content by weight greater than 31%,by afterwards carrying out a gelatinisation of the steeped rice byheating it to a temperature greater than 130° C. while maintaining themoisture at a weight content greater than 22% for a sufficient period,for example a period of at least 8 minutes, to obtain core-gelatinisedrice, and by carrying out gelatinisation by causing the steeped rice topass continuously into an enclosure so as to limit, by virtue of theequilibrium which is established within the enclosure between themoisture content of the rice which is undergoing gelatinisation and themoisture content of the gaseous atmosphere of the enclosure, thequantity of vapour emitted from the rice during gelatinisation, in orderto maintain the moisture content by weight of the rice in the course ofgelatinisation at a value greater than 22%, and by heating up thesteeped rice in the enclosure essentially by conduction so as not toincrease the moisture content by weight of the gelatinised riceappreciably, and in particular so as not to exceed a content of 28%.

When carrying out a standard parboiling in water vapour it is found thatthis type of rice cooks more rapidly than the other types.

The grains of parboiled rice have the following property: the cookedgrains, placed in a hermetically sealed receptacle and put in therefrigerator at 4° C. for 6 days, have a firmness measured in thevisco-elastograph equal to or less than 120% of that which they have 15minutes after cooking. This property is very much sought-after,particularly when a dish has to be prepared in advance. The grains donot harden upon cooling.

The following Examples illustrate the invention.

EXAMPLE 1

600 kg of perfumed Khao Dawk Mali cargo rice are placed in a vat andcovered by 1,500 litres of water at 65° C. The rice is left in contactwith the water for 3 hours. The steeping water is then discharged, andthe rice is extracted from the vat by gravity and drained. Its moisturecontent is then equal to 31.4%. It is then conveyed at a rate of flowfrom 100 to 600 kg/h (in accordance with various trials which have allgiven satisfaction) to an enclosure constituted by a coil with a lengthof 64 m and an internal diameter of 110 mm, the walls of which have beenheated up beforehand to 190° C. The rice is transported from the inletto the outlet of the enclosure by vibration at a speed of 7 m/minute.Its dwell-time is 8 minutes. Its temperature that is attained at theoutlet of the enclosure is 130° C. Its moisture content is 25.5%. Thecargo rice that has been treated in this way is then transported tocolumns for drying in hot air, where its moisture content is broughtback to 12.5% for its preservation. It is then machined in accordancewith the traditional process.

The rice that has been treated in this way and subjected to the alkalitest exhibits 98% of completely translucent grains, that is to saycompletely gelatinized grains.

The alkali test is carried out as follows.

100 parboiled grains of rice are placed in a 1.7% solution by weight ofKOH for 20 hours.

After 20 hours of reaction the degradation sustained by the grains isobserved:

intact grains: non-gelatinised grains

grains dispersed in an opaque chalky mass: only slightly gelatinisedgrains

completely translucent dispersed grains without chalky opaque core:totally gelatinised grains.

The density of the grain is equal to 95% of the density of the samegrain that has been machined but not treated. No fissure is visible tothe eye or to the binocular magnifier.

This rice cooks for 5 minutes in boiling water and, cooled for 15minutes in accordance with the protocol described above, exhibits thefollowing texture characteristics which are measured in thevisco-elastograph;

Fmean=59.0; standard deviation of 6 measurements=1.2.

The Chopin visco-elastograph (alveograph) permits the visco-elasticcharacteristics of the cooked rice to be evaluated by assessing itsdeformation under the application of a constant force of 700 g for 20seconds and then relaxation of the force for 40 seconds.

For the measurement in the visco-elastograph, 100 g of rice are cookedin one litre of boiling water. At the end of cooking, the grains aredrained for one minute on a sieve with a mesh size of 1.25 mm and arethen left to cool for 15 minutes in a 60-mm Petri dish which is placedan a moist sponge plate and covered by an 80-mm Petri dish in such a waythat the latter creates a seal with the water which saturates the spongeplate. Three grains that have been cooled in this way are then placed onthe visco-elastograph. The thickness E of the grains before applicationof the force and the thickness e₁ after crushing are noted. From thesevalues the firmness F=100·(e₁/E) is calculated. Six measurements arecarried out in respect of the same cooking, in order to calculate themean and the standard deviation of the six results obtained.

EXAMPLE 2

A batch of grains of rice of the variety Khao Dawk Mali having anamylose content of 13.4% is parboiled by the same process as in Example1, cooked for 10 minutes, drained for one minute, placed in ahermetically sealed receptacle and then put in the refrigerator at 4° C.for 6 days. The firmness of the grains is measured with the aid of thevisco-elastograph 15 minutes after cooking and after six days in therefrigerator.

15 minutes after cooking: F=34.7

after 6 days at 4° C.: F=38.8

The firmness of the grains after six days at 4° C. is equal to 112% ofthat which was measured 15 minutes after cooking.

COMPARATIVE EXAMPLE 1

Unlike Example 2, a batch of grains of rice of the same variety KhaoDawk Mali that has not been parboiled but has simply been blanch-cookedfor 11 minutes and kept for six days at 4° C. has a firmness equal to171% of the firmness measured 15 minutes after cooking.

15 minutes after cooking: F=37.9

after 6 days at 4° C.: F=64.8

COMPARATIVE EXAMPLE 2

In the same manner, a batch of grains of rice of the variety Thai Bonnethaving an amylose content of 26%, steamed as in Example 1 and cooked forten minutes and then kept for six days at 4° C., has a firmness equal to161% of the firmness measured after cooking.

15 minutes after cooking: F=48.3

after 6 days at 4° C.: F=77.8

COMPARATIVE EXAMPLE 3

1 kg of cargo rice Khao Dawk Mali having an amylose content equal to13.4% is steeped at 65° C. for 3 hours, then drained and subjected to asteam treatment under a pressure of 1 bar for 5 min. The moisturecontent of the rice that has been treated in this way is greater than31%. Afterwards the rice is dried and then machined. This steamed ricehas a cooking-time of 8 min.

COMPARATIVE EXAMPLE 4

1 kg of cargo rice Thai-Bonnet having an amylose content equal to 27%and treated in the same manner as in Comparative Example 3 has acooking-time of 10 min.

What is claimed is:
 1. Parboiled grains of rice that may be rapidlycooked in boiling water and are free of fissures visible to the unaidedeye, said grains having an amylose content lower than 15% by weightrelative to dry matter and being gelatinized by parboiling so that atleast 98% of said grains when placed in a 1.7% solution by weight of KOHfor 20 hours are completely translucent grains.
 2. The grains of rice ofclaim 1, wherein said parboiled grains of rice may be fully cookedwithin five minutes in boiling water.
 3. The grains of rice of claim 1,wherein said amylose content is between 10% and 15% by weight.
 4. Cookedgrains of rice having a reduced firmness after refrigerated storage areprepared from parboiled rice grains having an amylose content lower than15% by weight relative to dry matter and being substantially gelatinizedduring the parboiling, said cooked grains of rice, when placed in ahermetically sealed receptacle and maintained at 4° C. for 6 days,having a firmness in the visco-elastograph equal to or less than 120% ofthe firmness of the grains of rice 15 minutes after cooking, wherein thefirmness 15 minutes after cooking and the firmness after maintaining thegrains at 4° C. for 6 days is measured as follows: three grains of riceare placed on the visco-elastograph and the thickness E of the grainsbefore application of a force and the thickness e₁ after crushing bysaid force are noted and the firmness is calculated as 100·(e₁/E). 5.Cooked grains of rice as set forth in claim 4, wherein said parboiledrice is fully cooked within 5 minutes in boiling water.
 6. A process forpreparing parboiled rice that is suitable for rapid cooking comprising:a) steeping rice with an amylose content lower than 15% by weightrelative to dry matter in water at a temperature lower than 70° C.,under such conditions and for such time that, after removal of thesurface water by drainage or spin-drying, steeped rice is obtainedhaving a moisture content by weight greater than 31 percent; b)parboiling the rice from step (a) to gelatinize the rice so that atleast 98% of the rice grains when placed in a 1.7% solution by weightKOH for 20 hours are completely translucent grains; and c) drying therice from step (b).
 7. The process of claim 6, wherein step (a) includessteeping rice at a temperature in the range of from 40 to 70° C.